Magnetic recording and/or reproducing system

ABSTRACT

In reducing crosstalk between the low frequency, frequency-converted chrominance portions of h-aligned video signals on adjacent tracks, the frequency converting carrier, but not the entire frequency converted portion, has its polarity inverted at the end of each line interval during the recording of alternate tracks but not during the recording of the remaining alternate tracks. This eliminates the extraneous signal that would otherwise be produced in the balanced modulator of the playback apparatus by any direct voltage offset or transient remanent of a direct voltage offset in the recorded signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of magnetic recording andreproducing systems for video signals and particularly to the type ofsystem in which there is a polarity reversal of a certain portion of thesignal at the end of each line interval of certain groups of lineintervals.

2. The Prior Art

This system is related to the system described in copending U.S.application Ser. No. 492,330, filed July 26, 1974, and assigned to theassignee of the present application. In that system it was proposed toreduce the crosstalk interference of the low frequency portions of thevideo signals recorded on adjacent tacks in h-alignment by reversing thepolarity of the chrominance components at the end of each line intervalof alternate tracks but not to reverse the polarity at the end of lineintervals during the remaining alternate tracks. As a result, thecrosstalk component picked up during playback of each line intervalwould have a polarity that was either the same as or opposed to thepolarity of the main, or desired, chrominance component signal. Thechrominance components and cross-talk signals of successive lineintervals were then combined in a comb filter, which is a type of filterthat includes delay means and a subtractor circuit to combine, inopposite polarity, the signal applied to the delay means with the outputsignal of the delay means. The length of the delay is one line intervaland so the chromiance signal for each line interval is combined inopposite polarity with the chrominance signal of the succeeding lineinterval. During the recording of alternate tracks the chrominancecomponents of successive line intervals are recorded in oppositepolarity so that when they are combined subtractively, the alternationin polarity cancels out and the chrominance components of successiveline intervals then return to the same polarity and are added. However,the cross-talk signals, when passed through this same comb filter emergein successively opposite polarities and so are cancelled out or, atleast, are reduced.

For those tracks in which the successive line intervals of thechrominance signal are not reversed in polarity, switching means areprovided in the playback apparatus to select, during alternate lineintervals, chrominance components of opposite polarity so that, asapplied to the comb filter, they do have the required successiveopposite polarity condition. Again, the crosstalk signals reproducedalong the desired chrominance signals of the latter tracks are affectedby the same switching and comb filter arrangement so that they arecancelled or are at least substantially minimized.

The comb filter not only provides means for combining successive lineinterval signals, but also has a filtering effect that results in thesubstantially complete attenuation of signals having integral multiplesof the fundamental frequency that is delayed by one cycle in passingthrough the delay means. In the case of delay means having a delay ofone line interval, thus fundamental frequency is the basic linerepetition frequency of the system.

The effect of inverting the polarity of chrominance components duringsuccessive line intervals is to produce a frequency offset. In thesimplest terms, if a sine wave signal having the frequency f_(s) of thechrominance signal carrier were periodically inverted at a repetitionfrequency f_(h), which may conveniently be understood to be the basicline repetition frequency, the resultant modified signal would not havethe frequency f_(s) any more but, by Fourier analysis, would be seen tobe the combination of sinusoidal signals having frequencies f_(s)+1/2(f_(h)) and f_(s) 1/2(f_(h)). By choosing the frequency f_(s) to benf_(h) +1/2(f_(h)) where n is an integral number, the signal having thefrequency f_(s) could pass through the comb filter, but the signalshaving the frequencies f_(s) ±1/2(f_(h)) would not pass through the combfilter. This provides further separation of the desired signals, whichmay be the f_(s) signal and its side bands spaced from it by mf_(h),from the undesired signals, which have frequencies f_(s) ±1/2(f_(h))with side bands spaced mf_(h) therefrom. The number m is an integer andusually is much smaller than n. The side bands of the desired signalinterleave with side bands of the undesired, or crosstalk, signal andare all at frequencies to be separated from the undesired signal and itsside bands by the frequency response of the comb filter as well as bythe subtractive combination of successive line interval signals in thecomb filter.

The circuit that achieves the desired switching of polarity of alternateline interval signals of the chrominance signal in the above-mentionedprior application may inadvertently and undesirably introduce a voltageoffset. This is due to the fact that the signal of one polarity may havea certain DC axis and the signal of the other polarity may have adifferent axis so that when alternate line interval segments of thesetwo signals are combined, the DC axes come through the switchingoperation as a square wave having a voltage magnitude equal to thedifference in the DC axes. Even if the switched signal is passed througha filter to remove DC components, switching transients are still likelyto remain. For example, if the filter is simply a series capacitor, theleading edge of the square wave component will pass through unattenuatedand the level portion of the square wave component will decreaseexponentially in each cycle. The difficulty of removing the undesiredcomponents, or transient remanents, by filtering is increased due to thefact that the chrominance components are typically converted to afrequency band of about 687 KHz, but their bandwidth is such that theyextend ± 500 KHz from the 687 KHz figure. Thus the filter would have toeliminate DC signals but pass all signals between approximately 187 KHzand 1,187 MHz.

It is one of the objects of the present invention to provide an improvedsystem for eliminating the direct voltage offset in a system generallyof the foregoing type.

Further objects will be apparent from the following specificationtogether with the drawings.

SUMMARY OF THE INVENTION

According to the present invention, the entire chromimance componentsignal is not subjected to polarity inversion during alternate lineintervals. Instead, only the carrier has its polarity inverted. Thepolarity is inverted during selected line intervals before being used toconvert the frequency of the original chrominance component signal fromthe relatively high chrominance sub-carrier frequency f_(s) which, inthe NTSC-system is about 3.58 MHz, to the relatively low frequencyconverted frequency of about 587 KHz.

In the playback portion of the system, which may be constructedseparately from the recording portion or may be constructed as part of acombined recording and playback device, the polarity of the frequencyreconverting carrier is inverted during selected line intervals toachieve the necessary reconversion of the chrominance components totheir required relatively high frequency band around 3.58 MHz carrier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified representation of a short section of magnetictape showing the arrangement of recording of several tracks divided intoline intervals in h-alignment.

FIGS. 2A and 2B show the operative recording portion of two transducersfor recording the tracks in FIG. 1.

FIG. 3 is a block diagram of a prior art recording system in which thepolarity of selected line intervals of the frequency convertedchrominance signals are inverted.

FIG. 4 is a simplified representation of a short section of magnetictape illustrating the relationship between the polarities of the desiredchrominance signals and crosstalk signals as recorded by the apparatusin FIG. 3.

FIG. 5 is a block diagram of a prior art playback system for reproducingsignals recorded by the apparatus in FIG. 3.

FIGS. 6A to 6C show waveforms used in the recording and playbackapparatus in FIGS. 3 and 5.

FIGS. 7A to 7G are a series of graphical representations of desired andundesired chrominance signals, illustating interleaving of the undesiredsignals with the desired signals.

FIGS. 8A to 8C are a series of waveform diagrams illustrating the effectof direct voltage offset of the chrominance signals.

FIG. 9 is a block diagram showing both recording and reproducingapparatus constructed according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The short length of tape 11 shown in FIG. 1 has six tracks 12-17recorded on it. There tracks are shown as being recorded in abuttingrelationship, and the tracks are shown divided into small subsections,each of which represents the small area on which the entire video signalcorresponding to one line of a complete television image is recorded.The smaller sections at the ends of the tracks represent half-lineintervals for interlaced scanning.

The lines marking the ends of each of the subsections in each of thetracks 12-17 may be considered to represent the locations at which thehorizontal synchronizing signals are recorded. The recording is said tobe h-aligned since the horizontal signal, sometimes referred to as the hsignals, are recorded in alignment with corresponding signals onadjacent tracks. This is a well-known technique for reducing the type ofcrosstalk that would otherwise occur between adjacent tracks if therecorded horizontal synchronizing signals were not aligned.

The lines representing the location of recording of the horizontalsynchronizing signals in the tracks 12, 14, and 16 are represented asbeing perpendicular to the longitudinal direction of such tracks whereasthe lines representing the location of recording of horizontalsynchronizing signals in the tracks 13, 15, and 17 are at a differentangle with respect to the longitudinal direction of those tracks. Thisdifference in angle is produced by the air gap in the recordingtransducers as shown in FIGS. 2A and 2B. The air gap g₁ in thetransducer 19 in FIG. 2A has an angle θ₁ with respect to the linerepresenting the direction of movement of the tape relative to thetransducer 19. The angle θ₁ is represented as a right angle and thus thetranducer 19 would be used to record the tracks 12, 14 and 16. Thetransducer 21 in FIG. 2B has an air gap g₂ at an angle θ₂ with respectto the line representing the direction of relative movement between thetape and the transducer. The transducer 21 is the one that would be usedto record the tracks 13, 15, and 17. The angles θ₁ and θ₂ are known asthe azimuth angles, and it is not necessary that either of them beperpendicular to the direction of relative movement between thetransducer and the tape.

The recording of information at different azimuth angles reduces crosstalk between adjacent tracks not only from horizontal synchronizingsignals but also from other signals. In order to pick up the highestfrequency components recorded on a magnetic medium it is important thatthe azimuth angle of the reproducing transducer correspond exactly tothe azimuth angle of the transducer used to record that information. Anydiscrepancy in the azimuth angles of the recording and reproducingtransducers reduces the highest frequency signals that could otherwisebe reproduced. Deliberately choosing widely different azimuth angles inrecording adjacent tracks 12-17 in FIG. 1 substantially reduces anycross-talk from high frequency, and even medium frequency, componentsrecorded on adjacent tracks. Only the cross-talk between relatively lowfrequency components remains a problem.

The aforesaid prior application provided several techniques to reducecross-talk of low frequency components between adjacent tracks, eventhough the tracks were recorded in abutting or even slightly overlappingrelationship. FIG. 3 shows a block diagram of one type of recordingapparatus described in the aforesaid prior application.

In FIG. 3 a composite video signal is applied to an input terminal 22.From there the signal branches out into four paths one of which leads toa low pass filter 23 that passes luminance signal components upto about2.5 MHz or so. The output of the low pass filter is applied to a delaycircuit 24 that equalizes the signal delay in other parts of thebranched circuit. The luminance signal output of the delay circuit 24 isconnected to a frequency modulator 26 to frequency modulate a carriersignal in accordance with standard video tape recording practice. Theoutput signal of the frequency modulator is filtered by a high passfilter 27 and applied to a mixing circuit 28.

The composite video signal is also applied to a comb filter 29 whichpasses the chrominance signal components to a balanced modulator 31. Anoscillator 32 is also connected to the balanced modulator 31. Themodulator 31 has two output terminals connected to the fixed terminalsof a single-pole double-throw switch, or selecting device 33 and the armof this switch is connected to a low pass filter 34 which is connected,in turn, to the mixer 28.

The composite video signal is also supplied from the input terminal 22to a horizontal synchronizing, or sync, signal separator 36 and to avertical sync signal separator 37. The horizontal sync separator 36 isconnected to a flip-flop 38 and the vertical sync separator 37 isconnected to a flip-flop 39. Both of these flip-flops are connected toan AND gate 41 the output of which is connected to control theswitching, or selecting, circuit 43. The flip-flop 39 is also connectedto a servo-circuit 43 and to a control signal transducer 44 to recordcontrol signals along one edge of the tape 11.

The tape 11 wrapped helically part of the way around a drum 46. Thisdrum comprises an upper portion 47 and a lower portion 48 with a slot 49therebetween. The two transducers 19 and 21 are located at opposite endsof an arm 51 affixed to the end of a shaft 52 driven by a motor 53. Themotor is controlled by the servo-circuit 43. An amplifier 54 connectsthe mixer 28 to the transducers 19 and 21. The recording apparatus alsoincludes a servo-circuit 56 connected to the motor 53 to control theoperation of the motor and connected to the output of the flip-flop 39to be controlled by signals therefrom. The flip-flop 39 is alsoconnected to a fixed transducer 57 to record the output pulses of theflip-flop along one edge of the tape 11 to serve as control pulses togovern the speed of the tape during playback.

In the operation of the apparatus shown in FIG. 3, the oscillator 32generates a signal having a fixed frequency f_(c) =f_(s) +f_(a), andthis signal combines, in the balanced modulator 31, with the chrominancesignal components that pass through the comb filter 29. The balancedmodulator 31 subtracts the frequencies of the signals supplied thereto,produces two output signals indicated as C_(a) and -C_(a) which are ofopposite polarity. Each of these signals has the same frequencyconverted carrier frequency f_(a), when considered instantaneously, andthey are selected alternately by the switching circuit 33 to be appliedto the low pass filter 34 that eliminated undesired side bands andapplies only the proper frequency converted chrominance component signalto the mixer 28.

The operation of the switching circuit 33 to select either signal C_(a)or signal -C_(a) is controlled by the AND gate 41 in response to outputsignals from the flip-flops 38 and 39. The selected pattern of recordingof the signals C_(a) and -C_(a) is illustrated in FIG. 3 which shows ashort length of the tape 11 with two adjacent tracks 58 and 59 recordedon it. The track 58 is shown with four line areas, or increments 61-64and the track 59 is shown with four line areas, or increments, 66-69h-aligned with the adjacent line areas 61-64 respectively, on the track58. Each of the line areas 61-64 and 66-69 has two arrows in it, thelarger of which indicates the polarity of the frequency convertedchrominance component recorded therein, and the smaller of whichindicates the polarity of the cross-talk interference signal, which isthe frequency converted chrominance component signal in the nextadjacent line area of the adjacent track.

All of the frequency converted chrominance component signals recorded onthe track 58 have a carrier of the same polarity. This may be either thepolarity of the signal C_(a) or of the signal -C_(a). For the sake ofsimplifying the explanation it will be assumed that the polarity of thelarger arrows in the track 58 indicates that the signal C_(a) isrecorded in all of the line increments 61-64. In the track 59 thepolarity of the signal is reversed in alternate line areas ofincrements, that is, in line areas 66 and 68, the signal C_(a) isrecorded and in line areas 67 and 69 the signal -C_(a) is recorded.However, the effect of alternately switching back and forth between thesignals C_(a) and -C_(a) is not as simple as it seems. As will bedescribed hereinafter, the signal in the track 59 may be considered tobe a new signal C_(b) having frequency components offset with respect tothe components of the signal C_(a) (or -C_(a)) to interleave therewith.

In order to record the signals C_(a) and -C_(a) in the pattern set forthin FIG. 3, the simple logic circuit involving the AND gate 41 is used.FIG. 6A shows the output signal P_(h) of the flip-flop 38 as being asquare wave having high and low intervals, each having a duration of oneline interval, or 1h. One complete cycle of the signal in FIG. 6A thushas a fundamental frequency 1/2(f_(h)). The output signal of theflip-flop 39 is shown in FIG. 6B as a square wave P_(v) having high andlow intervals each equal to 1v, where v is a field interval.

Since the AND gate 41 can produce a high output only when both of theapplied signals P_(h) and P_(v) are high, the output of the AND gate asis shown in FIG. 6C, remains low during one entire field interval T_(a)and goes high only during alternate line intervals of the alternatefield interval T_(b). This is based on the assumption that each trackrecords one complete field interval. The pattern shown in FIG. 3corresponds to having the arm of the switching circuit 33 apply thesignal C_(a) to the low pass filter 34 when the output of the AND gate41 is low and having the arm apply the signal -C_(a) to the low passfilter 34 when the output of the AND gate 41 is high.

FIG. 5 shows a playback apparatus for reproducing video signals recordedby the apparatus of FIG. 3. Many of the components in FIG. 5 areidentical with those in FIG. 3 and such identical components areindicated by the same reference numerals as in the earlier figures anddescriptions of such elements. The description of their operation willnot be unnecessarily repeated.

The reproduced signals from the transducers 19 and 21, which are alsoused in playing back recorded signals, are amplified in an amplifier 71and are applied to a high pass filter 72 and a low pass filter 73. Thehigh pass filter 72 passes the frequency modulated signal that includesthe luminance components. This signal is limited in a limiter 74 anddemodulated in a demodulator 76. The re-created luminance signal is thenamplified in an amplifier 77 and applied to a mixer 78.

The frequency converted chrominance signal separated by the low passfilter 73 is applied to the balanced modulator 31 along with a signalfrom an oscillator 79. The signal from the oscillator 79 has a frequencyf_(c) =f_(s) +f_(a) and is constant during all line and field intervals.Two output terminals of the balanced modulator 31 are connected to thefixed terminals of the switching circuit 33, and the output of thelatter is applied to a comb filter 81. The output of the comb filter isconnected to the mixer 78 and to a burst gate 82. The burst gate and theoutput of an oscillator 83 are connected to a phase comparison circuit84 that is connected to the oscillator 79. A waveform circuit 86, whichmay be a rectifier, is connected to the transducer 57 to receivereproduced control signals therefrom, and its output is connected to aresetting terminal of the flip-flop 39.

The operation of the system in FIG. 5, insofar as the chrominancecomponent signal is concerned, consists in applying the signal havingthe frequency f_(c) =f_(s) +f_(a) from the oscillator 79 to the balancedmodulator 31 to convert the frequency f_(a) of the signals C_(a) andC_(b), which are applied alternatively to the balanced modulator 31 backto the original chrominance carrier frequency f_(s). The two outputterminals of the balanced modulator 31 provide signals of oppositepolarity. One of them includes the desired signals C_(sa) and theundesired or cross-talk signal C_(sb) ', while the other includes thedesired signal -C_(sa) and the undesired or cross-talk signal -C_(sb) '.The designation C_(sa) indicates that the carrier frequency of thefrequency converted chrominance signal C_(a) has been reconverted to theoriginal frequency f_(s). The designation C_(sb) ' indicates that thesignal C_(b), which consisted of alternate line intervals of the signalsC_(a) and -C_(a) has been reconverted by the same converting signalhaving the frequency f_(c) =f_(s) +f_(a).

The switching circuit 33 is controlled by the AND gate 41 to produceexactly the same switching pattern as is shown in FIG. 6C. The waveformcircuit 86 assures that the operation of the flip-flop 39 in theplayback unit properly relates to the operation of the flip-flop 39 inthe recording system of FIG. 3.

The output of the switching circuit 33 is applied to the comb filter 81.It will be recalled that the comb filter includes both a direct signaland a path in which the signal is delayed by one horizontal lineinterval. The output of the direct path is combined in the delayedoutput of the other path. Thus, when the chrominance component signalsof the track 58 in FIG. 4 are being reproduced, the desired reconvertedchrominance component signals C_(sa) corresponding to the signals C_(a)indicated by the long arrows in two successive line areas 61 and 62 or62 and 63 or 63 and 64 are combined, with the polarities of theircarriers being the same, at the output of the comb filter. However, theundesired, or cross-talk, components C_(sb) ' corresponding to thesignals C_(b) ' indicated by the small arrows in the line incrementshave carriers of opposite polarities in successive pairs of lines, andthus cancel each other when combined at the output of the comb filter81. As a result, the output signal of the comb filter 81 in FIG. 5during the reproduction of the track 58 consists substantially only ofthe desired chrominance components C_(s) having the proper carrierfrequency f_(s). During the reproduction of the track 58, the switchingcircuit 33 does not switch back and forth between its two inputterminals but remains on only one terminal as indicated during theinterval T_(a) in FIG. 6.

During the reproduction of the track 59, the switching circuit 33 doesswitch back and forth at the end of each line interval of time inaccordance with the output signal of the AND gate 41 during the intervalT_(b) as indicated by the long arrows in line areas 66-69 in FIG. 4. Theswitching signal is indicated in FIG. 6C. Thus, the comb filter 81receives the signals C_(sb) and C_(sa) ' during group of line intervalsrecorded along the track 59.

Considering the signals on a line-by-line basis, since the chrominancesignal components recorded in line areas 66 and 67 have oppositepolarities, inversion of the signal reproduced from line area 67 causesthe chrominance components signal to be combined, in phase, with thedelayed chrominance component signal reproduced from line area 66 at theoutput of comb filter 81. However, since the chrominance componentsignals are recorded in all line areas of the next adjacent track 58with carriers of the same polarity, the reconverted cross-talk signalsC_(sa) ' from track 58, which are reproduced with the chrominancecomponent signals recorded in the successive line areas of the track 59also have the same polarity. Therefore, the abovementioned inverting ofthe signal reproduced from line area 67 of track 59 causes thecross-talk signal C_(a) ' reproduced with the signal recorded in linearea 67 to be combined, with its phase or polarity reversed, with thedelayed cross-talk signal reproduced with the signal recorded in linearea 66, whereby the combined cross-talk signals cancel each other atthe output of comb filter 81.

The reason why inversion of polarity of the signal C_(a) at the end ofeach line interval changes the signal frequency may be explained byconsidering a simplified situation in which signals C_(a) and -C_(a),both of which have the carrier frequency f_(a), are not modulated bychrominance components but are available at the two output terminals ofthe balanced modulator 31 in FIG. 3 as pure sine waves of oppositepolarity. During the field interval T_(b) when signals C_(a) and -C_(a)are selected alternately by the switching circuit 33, the output signalof the switching circuit is no longer a single signal but is a sine wavewhose polarity reverses, or whose phase shifts 180°, at a repetitionrate of 1/2(f_(h)). When a Fourier analysis is made of such a signalover a complete cycle of the interval of two horizontal lines, it willbe found that the carrier frequency f_(a) is no longer present, but hasbeen replaced by first upper and lower side bands spaced by ±1/2(f_(h))from the original carrier frequency and by additional upper and lowerside bands spaced from the first mentioned side bands and from eachother, in order, by f_(h). Therefore, in effect, the signal-pole,doublethrow switching circuit 33 operates as a balanced modulator, andthe modulating signal is the switching signal P_(k) in FIG. 6C. Duringthe interval T_(b), this signal changes its level at a rate that takestwo horizontal line intervals for a complete cycle and therefore has afrequency of 1/2(f_(h)). Being, in effect, a balanced modulator, theswitching circuit 33 produces a balanced output signal without acarrier. This balanced output signal, since it interleaves with thesignal C_(a) may be referred to as the signal C_(b), and thus there is,in fact, an interleaving relationship between the carriers of thefrequency converted carrier components of the signal recorded on thetrack 58 and that recorded on the track 59 in FIG. 4. Such interleavingrelationship provides for an interleaving relationship between thepreviously referred to cross-talk or interference signals C_(sb) and-C_(sb) and the desired signals C_(s) which further improves thecancellation of the cross-talk signals.

FIGS. 7A to 7G show the interleaving frequency relationship of thechrominance signals in the circuits in FIGS. 3 and 5. FIG. 7A shows aportion of the spectrum of the frequency converted signal C_(a) whichcomprises a central carrier frequency f_(a) with principal harmonicsspaced from it ±nf_(h) and with subsidiary harmonics spaced from thecarrier frequency f_(a) and from each of the principal harmonics by thefield repetition frequency of the system. The signal C_(a) is generatedin the balanced modulator 31 in FIG. 3 during the recording of the track58 in FIG. 4.

FIG. 7B shows a spectrum similar to that in FIG. 7A, except that itscomponents are offset 1/2(f_(h)) with respect to the frequencies in FIG.7A. The signal in FIG. 7B is the desired chrominance signal C_(b)recorded in the track 59 in FIG. 4.

As indicated by the double arrows in each of the line interval areas inthe tracks 58 and 59 in FIG. 4, each of the desired chrominance signalsis unavoidably mixed with a cross-talk signal. These cross-talk signalsare illustrated in the spectra in FIGS. 7C and 7D which correspond,respectively, to the spectra in FIGS. 7A and 7B. In FIG. 7C thecross-talk signal is actually an attenuated version of the signal C_(b),and is therefore designated as C_(b). In FIG. 7D the cross-talk signalis an attenuated version of the signal C_(a), and is thereforedesignated as C_(a) '.

FIGS. 7E and 7F show the spectra of the chrominance signals at theoutput of the switching circuit 33 in FIG. 5. Although the signals C_(a)and C_(b) are converted in the balanced modulator 31 by the signal f_(c)=f_(s) +f_(a) from the oscillator 79, and, as converted, are designatedas signals C_(sa) and C_(sb), the fact that the arm of the switchingcircuit is held fixed in one position during the playback of the track58 in FIG. 4 but is switched from one of its positions to the other atthe end of each line interval during the playback of the track 59 inFIG. 4, results in eliminating the 1/2(h) offset of the signal C_(b).Thus, the reconverted signals C_(sa) and C_(sb) both have the samecarrier frequency f_(s), which is the original chrominance sub-carrierfrequency of the television system. In the spectra shown in FIGS. 7E and7F the undesired cross-talk signals C_(sa) ' and C_(sb) ' spaced midwaybetween the principal side bands of the desired signals C.sub. sa andC_(sb) and can be eliminated by the comb filter 81 to yield the desiredsignal C_(s), which is shown in FIG. 7G and is free of cross-talkcomponents.

FIG. 8A shows the waveform of several line intervals of the chrominancesignal C_(a) and -C_(a) and accompanying burst signal B_(a) and -B_(a)at the output of the switching circuit 33 in FIG. 3. There is a DCoffset of alternate line interval signals due to the fact that the inputterminals of the switching circuit 33 are connected to points in thecircuit of the balanced modulator 31 that have different DC components.Although the DC offset is illustrated as if it were positive for theline intervals in which the signal C_(a) is selected and relativelynegative for the remaining line intervals when the signal -C_(a) isselected, the polarity of the offset could be reversed. Passing thesignal shown in FIG. 8A through the low pass filter 34 reduces the DCcomponent and results in the signal shown in FIG. 8B. However, thissignal still has an initial offset 86 or 87 at the beginning of eachhorizontal line interval when the switching of the circuit 33 takesplace. When this signal with the offsets 86 and 87 is recorded on thetape 11 and is then played back by means of the circuit shown in FIG. 5,the carrier signal having the frequency f_(c) is modulated in thebalanced modulator 31 not only by the relatively high frequencychrominance components ±B_(a) and ±C_(a) of the frequency convertedchrominance and burst signals, but also by the DC offset components 86and 87. This is due to the fact that the balanced modulator 31 in FIG. 5produces an output signal based on its carrier freqency f_(c) wheneverthe input signal from the filter 73 differs from zero. As a result, theoutput signals of the balanced modulator 31 in FIG. 5 as shown in FIG.8C not only have the frequency reconverted chrominance portions C_(sa)during the visible part of each line interval and B_(sa) during thebursts, but also have undesired signals 88 and 89 of the frequency f_(c)=f_(s) +f_(a) and of amplitude determined by the remanent of DC offset86 and 87 of the switching signal as recorded on the tape 11.

FIG. 9 shows an embodiment of the present invention including bothrecording and playback sections. The recording section includes manycomponents found in the recording apparatus shown in FIG. 3 and theplayback section includes some components found in the playbackapparatus of FIG. 5. The description of these components and theiroperation will not be unnecessarily repeated.

Between the input terminal 22 and the horizontal and verticalsynchronizing separators 36 and 37 are two double throw switches 91 and92. The arm of another double throw switch 93 is connected to thetransducers 19 and 21. The arm of each of the switches 91-93 makescontact either with a pole identified R or a pole identified P,depending upon whether the apparatus is to be used for recording orplayback. In practice the arms of the three switches 91-93 would bemechanically linked together to operate as a three-pole double-throwswitch.

The chrominance components of the video signal applied to the inputterminal 22 to be recorded are separated out by the comb filter 19 andapplied to a frequency converter 94. This frequency converter alsoreceives signals that originate in an oscillator 96 and are amplified ina differential amplifier 97 that has two output terminals of oppositepolarity. These output terminals are connected to two fixed terminals ofa switching circuit 98, and the arm of the switching circuit isconnected through a high pass filter 99 to the frequency converter 94.The output terminal of the AND gate 41 is connected to the actuatinginput terminal of the switching circuit 98.

In the playback section of the apparatus in FIG. 9, reproduced signalsamplified by the amplifier 71 and filtered by the low pass filter 73 areconnected to another frequency converter 101 that also receives signalsfrom the high pass filter 99. The output of the frequency converter 101is connected through a high pass filter 102 to the comb filter 81. Inthe operation of the apparatus of FIG. 9 the composite video signalapplied to the input terminal 22 is separated by the low pass filter 23and the comb filter 29 into luminance and chrominance components,respectively. The luminance components are applied to a frequencymodulator 26 and the resulting frequency modulated signal is applied tothe mixing circuit 28.

The chrominance components pass through the filter 29 and are applied tothe frequency converter 94, have a carrier frequency f_(s), which forNTSC signals, is approximately 3.58 MHz. The oscillator 96 produces asignal having a frequency f_(c) =f_(s) +f_(a). This signal is amplifiedby the amplifier 97 and positive and negative polarity versions of thissignal are passed, in a predetermined sequence, through the switchingcircuit 98. The resulting signal is filtered by the high pass filter 99and applied to the carrier frequency input terminal of the frequencyconverter 94.

The sequence in which positive and negative versions of the signalhaving the freequency f_(c) are passed through the switching circuit 98is controlled by the output signal of the AND gate 41. This signal isshown in FIG. 6C. During the interval T_(a) in FIG. 6C, the switchingcircuit 98 passes only one version of the signal, either the positive orthe negative version. During the next interval T_(b),, corresponding tothe next recorded track on the tape 11, the switching circuit 98 wouldalternate back and forth between the positive and negative polaritysignals. For the reason given previously, the resulting signal passedthrough the high pass filter 99 during the interval T_(b) in FIG. 6Cwould not only reverse polarity at the end of each horiziontal lineinterval but would actually shift to a frequency relationship that wouldinterleave with the basic frequency f_(c) generated by the oscillator96.

Contrary to the arrangement shown in FIG. 3 in which the entirechrominance signal is applied to the mixing circuit 28 in eitherpositive or negative polarity, only the carrier signal is applied to thefrequency converter 94 in either positive or negative polarity. It isrelatively easy to eliminate any DC component of the signal of theoutput of the switching circuit 98 by means of the filter 99, and thereis no DC offset produced in the frequency converter 94. However, thereis still the desired reversal of polarity of the frequency convertedcarrier during certain intervals as determined by the switchingoperation depicted in FIG. 6C, and there is also the frequencyinterleaving relationship between components of the frequency convertedsignal produced during the interval T_(a) in FIG. 6C and that producedduring the interval T_(b). As a result, all of the advantages heretoforedescribed with respect to the recording apparatus shown in FIG. 3 areretained, but the disadvantage of having a DC offset is eliminated.

The resulting chrominance signal is combined in a mixing circuit 28 withthe frequency modulated signal that includes luminance information, andthe combined signal is passed through the switch 93 to be recorded bythe transducers 19 and 21 on the tape 11.

During playback of information previously recorded on the tape 11, thearms of the switches 91-93 are transferred to their P terminals. Thispermits signals picked up by the trnsducers 19 and 21 to pass throughthe switch 93 to the amplifier 71 and be separated into high frequencyand low frequency components. The high frequency components include theluminance information in frequency modulated form, and this informationis extracted by the demodulator 76 and applied through the amplifier 77to the mixing circuit 78.

The low frequency components that include the frequency convertedchrominance signal pass through the low pass filter 73 to the frequencymodulator 101. These components have a frequency converted carrier witha basic frequency f_(a). The converting carrier from the high passfilter 99 applied to the frequency converter 101 has a frequency f_(c),and the output of the frequency converter 101 thus has a carrier that isreturned to the original sub-carrier frequency f_(s). The chrominancecomponents grouped around the carrier at the frequency f_(s) are able topass through the high pass filter 102, and the desired components areseparated from the cross-talk components by the comb filter 101 inexactly the same way that the desired components and the cross-talkcomponents are separated in the circuit shown in FIG. 5. Since there isno DC offset in the recorded signal, the signals 88 and 89 shown in FIG.8C are not produced, and the reproduced composite signal at the outputterminal 80 is free of this undesired interference.

The invention has been described in specific terms but it will beunderstood by those skilled in the art that modifications may be madetherein. One such modification would be to derive the signal P_(v) shownin FIG. 6B from a transducer associated with the rotating shaft 52. Suchtransducers are known, and in this instance, the use of a signal P_(v)derived therefrom would have the advantage of causing the intervalsT_(a) and T_(b) in FIG. 6C to correspond exactly to the rotation of theshaft 52. Still further modifications may be made in the inventionwithin the scope of the following claims.

What is claimed is:
 1. Apparatus in which video signals are recorded inh-alignment in adjacent tracks on a recording medium, said apparatuscomprising:A. means for generating a frequency converting carrier signalin first and second versions of opposite polarity; B. a frequencyconverter to receive chrominance components of said video signals; andC. means to apply said first and second versions of said carrier signalin a predetermined sequence to said frequency converter, said sequencebeing such that, during alternate tracks, only one of said versions isapplied and, during the remaining alternate tracks, said first andsecond versions are applied alternately in successive line intervals. 2.The apparatus of claim 1 in which said carrier generating meanscomprises:A. an oscillator to generate said carrier signal; B. adouble-throw switching circuit comprising first and second inputterminals connected to receive said first and second versions of saidcarrier signal, said switching circuit comprising an output terminalthat can be conductively connected to said first and second inputterminals alternately; and C. a filter connecting said output terminalof said switching circuit to said frequency converter.
 3. The apparatusof claim 1 comprising:A. a low pass filter connected to said frequencyconverter; and B. transducer means connected to said filter to recordthe frequency converted signals on said recording medium.
 4. Theapparatus of claim 3 comprising:A. a second frequency converter aconnected to said means to apply said carrier signal, whereby saidsecond converter receives said first and second versions in saidpredetermined sequence; B. switching means to connect said transducermeans alternatively to receive frequency converted signals from saidfirst-named frequency converter and to supply reproduced signals fromsaid recording medium to said second frequency converter; and C. a combfilter connected to said second frequency converter to filterinterleaved cross-talk components from desired, frequency convertedchrominance signals of opposite polarity.
 5. The apparatus of claim 1comprising:A. reproducing transducer means to play back informationrecorded on said recording medium; and B. a comb filter connected tosaid frequency converter to filter interleaved cross-talk componentsfrom desired, frequency converted chrominance signals of oppositepolarity. .Iadd.
 6. Apparatus in which video signals are recorded inadjacent tracks on a recording medium, said apparatus comprising: A.means for generating a frequency converting carrier signal in first andsecond versions of opposite polarity; B. a frequency converter toreceive chrominance components of said video signals; and C. means toapply said first and second versions of said carrier signal in apredetermined sequence to said frequency converter, said sequence beingsuch that, during alternate tracks, only one of said versions is appliedand, during remaining alternate tracks, said first and second versionsare applied alternately in successive line intervals. .Iaddend..Iadd. 7.Apparatus for reproducing video signals that have been recorded inadjacent tracks on a recording medium with a carrier having one set ofphase conditions line interval by line interval in alternate tracks anda different set of phase conditions line interval by line interval inthe remaining alternate tracks, the phase conditions of carriers of thesignals recorded in adjacent tracks being such as to createpredetermined cross-talk relationships during reproduction, saidapparatus comprising: A. means to reproduce the signals recorded on eachtrack and cross-talk signals from adjacent tracks; B. means to shift thecarrier frequency and phase condition of the reproduced signals for aline interval at a time; and C. means to combine the resultantreproduced and revised signals and cross-talk signals with thereproduced and revised cross-talk signals of the next successive lineinterval to reduce the amplitude of said cross-talk signals..Iaddend..Iadd.
 8. The apparatus of claim 7 in which said means tocombine comprises a comb filter that comprises an input terminal, anoutput terminal, and parallel signal paths joining said input terminalto said output terminal, one of said paths comprising delay means todelay the passage of signals therethrough by a period of time equal toone horizontal line interval. .Iaddend..Iadd.
 9. Apparatus for recordingvideo signals in adjacent tracks on a recording medium and for playingback the recorded signals, said apparatus comprising:A. means forgenerating a frequency converting carrier signal in first and secondversions of opposite polarity; B. a frequency converter to receivechrominance components of said video signals; C. means to apply saidfirst and second versions of said carrier signal to said frequencyconverter in a predetermined sequence such that, during the recording ofalternate ones of said tracks, only one of said versions is utilized tofrequency convert said chrominance components and, during the remainingalternate tracks, said first and second versions are utilizedalternately for frequency converting successive line intervals; D. meansto record and reproduce said chrominance components line interval byline interval and track by track, said reproducing means alsoreproducing cross-talk chrominance signals from the next adjacent track;E. means to reconvert the frequency of the reproduced carrier in thereproduced chrominance components by means of a carrier signal havingthe same frequency as said frequency converting carrier signal; F. meansto control the phase of said carrier applied to said frequencyreconverter to correspond to said first and second versions of oppositepolarity according to a predetermined sequence; and G. a comb filterconnected to the output of said frequency reconverter to filter outcross-talk components of the frequency reconverted signal..Iaddend..Iadd.
 10. Apparatus in which video signals are recorded inadjacent tracks on a recording medium, said apparatus comprising: A.means for generating a frequency-converting carrier signal, the phase ofwhich changes in a predetermined manner at predetermined line intervals;B. a frequency converter to receive chrominance components of said videosignals; and C. means to apply said carrier signal to said frequencyconverter, the sequence of phase changes of said carrier signal beingsuch that, during the recording of adjacent tracks, the relative phaseof the carrier of the frequency converted signal has a predeterminedrelationship to the phase of the carrier of the adjacent recorded signalin the next adjacent track. .Iaddend..Iadd.
 11. Apparatus in which videosignals are recorded in adjacent tracks on a recording medium, saidapparatus comprising: A. means for generating a carrier signal the phaseof which is changed in a predetermined manner at the end of certainhorizontal line intervals; B. means for changing the frequency and phaseof chrominance components of said video signals in accordance with saidcarrier signal and to produce a frequency converted chrominance signal;and C. transducer means for recording said frequency convertedchrominance signal on said recording medium with a predetermined phaserelationship to minimize interference during playback of said frequencyconverted chrominance signals recorded in adjacent tracks..Iaddend..Iadd.
 12. Apparatus in which video signals are recorded inadjacent tracks on a recording medium, said apparatus comprising: A. achrominance signal source for supplying a chrominance signal having afirst carrier frequency; B. a carrier signal source for supplying acarrier signal having a second frequency higher than said firstfrequency, the phase of said carrier signal being changed in apredetermined manner at the end of selected line intervals forcancellation of cross-talk between signals from adjacent tracks duringreproduction of the recorded signals; C. frequency converter means forchanging the frequency of said chrominance signal to a third frequencycorresponding to the difference between said first frequency and saidsecond frequency in response to said carrier signal; and D. recordinghead means for recording the frequency changed chrominance signals inadjacent tracks on said recording medium. .Iaddend..Iadd.
 13. Apparatusfor reproducing chrominance signal recorded in adjacent tracks on arecording medium, the recorded chrominance signals on adjacent trackshaving different phase conditions, said apparatus comprising: A. meansfor reproducing said recorded chrominance signals; B. means forgenerating a carrier signal the phase condition of which is changed inresponse to the phase condition of the chrominance signal beingreproduced; C. means for frequency converting the reproduced chrominancesignal in accordance with said carrier signal; D. comb filter means; andE. means for supplying said frequency converted chrominance signal tosaid comb filter means. .Iaddend..Iadd.
 14. Apparatus for recordingvideo signals in adjacent tracks on a recording medium and forreproducing said signals, said apparatus comprising: A. a chrominancesignal source for supplying a chrominance signal having a first carrierfrequency; B. a carrier signal source for supplying a frequencyconverting carrier having a second frequency higher than said firstfrequency, the phase of said frequency converting carrier signal beingchanged in a predetermined manner at the end of selected line intervals;C. frequency converter means for changing the frequency of saidchrominance signal to a third frequency corresponding to the differencebetween said first frequency and said second frequency in response tosaid frequency converting carrier signal as changed; D. means forrecording the frequency changed chrominance signal in adjacent tracks onsaid recording medium and for reproducing said recorded frequencychanged chrominance signal; E. means for generating a frequencyreconverting carrier signal at said second frequency, the phase of saidreconverting carrier signal being changed at the end of selected lineintervals in a manner determined by said changed frequency convertingcarrier signal to cancel cross-talk between signals from adjacent tracksduring reproducing of the recorded signals; and F. comb filter meansconnected to receive the frequency reconverted chrominance signal toreduce the amplitude of undesired cross-talk signals therefrom..Iaddend.